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From wikipedia:

U.S. Treasury statistics indicate that, at the end of 2004, foreigners held 44 percent of federal debt held by the public. [3] About 64% of that 44% was held by the central banks of other countries. A large portion was held by the central banks of Japan and China. This exposes the United States to financial or political risk that either bank will stop buying Treasury securities—or start selling them heavily. In fact, the debt held by Japan reached a maximum in August of 2004 and has fallen nearly 3 percent since then. [4] This may change, of course, but it does underscore the risk.

Some who study geopolitics and creditary economics are greatly concerned by this. With strong financial ties, others believe that there is an incentive for the U.S. to be protective of the respective nations. For instance, military strategist Thomas Barnett believes that the world's nations are essentially paying the United States to be the world's policeman. There is a risk that military confrontations could occur between the U.S. and its debtors if the country cannot repay the debts, though most analysts believe that any confrontations would be entirely economic.

A risk of even greater magnitude is the possibility that OPEC will begin to price petroleum in Euros, as Saddam Hussein began to do in 1998—until this decision was reversed by the 2003 invasion of Iraq.

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